Shielded cable, also known as armored cable and BX, consists of a helically wound strip of metal that provides a relatively flexible duct in which conductor wires are protectively enclosed. For cutting such material to a desired length, a saw cut must be made through the helical strip, in a direction substantially lengthwise of the cable and having a length at least equal to the width of that strip.
Although a hack saw is a commonly used tool for cutting shielded cable, it is not well suited for the purpose because the cable must be rather sharply flexed at the cutting zone and the hack saw blade must be applied to a surface on the cable that curves away from the blade edge in all directions and thus encourages the saw to slip off of the cable and possibly injure the person performing the operation. In addition, the depth of a cut through the shielding should be carefully controlled to avoid damage to insulation on the conductors inside it, but there is no reliable way to gage the depth to which a hack saw blade has penetrated the shielding.
With these considerations in mind, a number of devices for cutting shielded cable have heretofore been proposed, intended to afford safety both for the user and for the insulation on the conductors. Most such devices have comprised a circular saw blade that was rotated by means of a hand crank or the like, cooperating with a cable supporting means whereby the shielded cable was secured in such relation to the saw blade that the latter could penetrate the shielding to only a limited depth.
U.S. Pat. No. 2,654,941 discloses such a device wherein the cable supporting means comprises two elongated lengthwise spaced apart cradle sections, one to underlie the cable and support it, the other to overlie the cable and steady it. The cable is maneuvered between the two cradle sections and then swung into engagement with them. Under some conditions a shielded cable could probably be attached to this device, cut and removed from it with commendable speed and facility. However, the cable is not positively clamped to the device, and the flexing of a free length of shielded cable is difficult to control, so that there are situations in which the cable would control the position and orientation of the cutting device, rather than being controlled by it, and the cutting operation would therefore be awkward. A further and very important disadvantage of this device is that it can be used with shielded cable of only one diameter.
U.S. Pat. No. 2,674,027 discloses a device wherein the shielded cable to be cut must be slid lengthwise in an elongated substantially tubular holder to bring it to a position at which a cut is to be made in it, and the cable is clamped at that position by means of a thumb screw threaded into the holder transversely to its length. The device is obviously inconvenient to use when a cut must be made at a location some distance from each end of a long piece of cable.
U.S. Pat. No. 3,453,917 discloses a device which is intended to be mounted on a workbench or the like and which therefore presents the inconvenience that, for every cut, the cable must be moved to the workbench or the workbench moved to the cable. The cable is clamped to the device by means of a thumb screw, so that securement and release of the cable are not convenient operations.
U.S. Pat. No. 3,633,275 discloses a device comprising hinged clamping jaws for confining the cable and a rotary cutter hinged to one of the jaws. The operator grips the jaws with one hand to exert manual clamping force upon the cable while attending to actuation of the rotary cutting blade with the other hand. Confinement and release of the cable can be effected rather quickly with this arrangement, but the security with which the cable is locked to the device is dependent upon the strength of the operator's one hand. Use of this device can therefore be fatiguing, especially where several cuts must be made in rather rapid succession. Furthermore, the clamping holder is inherently suited only for larger diameter shielded cables; to adapt it for smaller diameter cables a small spacer must be fastened to one of the clamping jaws by means of a thumb screw. Installation of such a spacer is in itself time consuming and inconvenient, and because it is a relatively small part it can be easily lost or misplaced. Another important disadvantage of this device is that the saw blade is biased towards the cable by means of a spring that exerts a predetermined force which the user cannot vary during the course of a cut and which may be too great under some circumstances and too small at other times.
U.S. Pat. No. 3,633,275 discloses a cable cutter comprising a pair of elongated handle members, each having a free end and a hinge end. They are hingedly connected at their hinge ends for limited swinging towards and from side-by-side relationship. A crank-operated rotary cutter is mounted near the free end of one of these handle members. A cable to be cut is lengthwise confined in the other one, which constitutes a cable carrier. The user operates the crank with one hand while holding the handle members in his other hand and squeezing them towards one another with a pressure that determines the cutting feed force of the cutter against the cable. This arrangement ensures that the actual cutting will take place smoothly, easily and quickly; but the device as disclosed does not offer optimum convenience. U.S Pat. No. 3,851,387 discloses a generally similar cable cutter wherein the cable carrier is in the form of a trough that is open at one side, for lateral insertion of the cable to be cut, and the cable is clamped in place by means of a thumb screw threaded into the cable carrier near its free end. The thumb screw has the disadvantage that it must be tightened into secure clamping engagement with the cable for a cutting operation, and then loosened afterward. These manipulations take time and require a certain amount of strength.
Indeed, in his later U.S. Pat. No. 4,359,819 the patentee of No. 3,851,387 says that "the use of a thumb screw for that purpose [holding the cable] is sometimes rather awkward and inconvenient". In place of the thumb screw, No. 4,359,819 discloses a clamping lever that is pivoted to the cable carrier along its side remote from the handle member on which the cutter is mounted. A spur-like plunger on this lever projects into the cable carrier for engaging the cable, clamping it with a force that depends upon the squeeze applied to the lever. As the patent says: "The manual compressive force which thus grips the cable firmly in the handle member . . . also presses the saw blade . . . against the cable surface . . . ." One disadvantage of this is that the clamping force applied to the cable tends to be variable and is not independent of the cutting force that the saw blade exerts against the cable. Another disadvantage is mentioned in the patent itself: "In the operation just described the cable holding power of the plunger depends somewhat on the manual strength of the operator which, in some cases, might be excessive." For minimizing these disadvantages the patent discloses certain adjustable expedients, but they complicate the structure, increase its cost and impose the need for making time consuming adjustments in more or less cut-andtry fashion for every different size of cable.
U.S. Pat. No. 4,697,343 to S. J. Collins discloses a cable cutter having a pair of elongated handle members and a rotary cutter, all arranged substantially like those in the last two patents discussed above. Cable to be cut is clamped into the cable carrier by means of a lever pivoted to it and having lobes that engage the cable. This lever swings about an axis that extends lengthwise along the cable carrier, and the several lobes have rounded tip portions at different distances from that axis. For securing a cable in the cable carrier, the lever is simply swung away from the orbit of the rotary cutter crank handle until one of the lobes clampingly engages the cable. This device is very fast and easy to use, and simple and inexpensive in construction, but its clamping lever, in projecting laterally from its handle members, presents inconveniences with respect to tool box storage and packaging for retail sale. There is also some possibility that a cable to be cut will not be securely clamped in its cable carrier, because the several clamping lobes have stepwise different radii, each being intended for cooperation with cable of a specific nominal diameter, whereas the outside diameter of any given shielded cable varies markedly from point to point along its length.
As pointed out in some of the above-discussed patents, a cable cutter of the type here under consideration is essentially a safety device intended to protect the user against the hazards inherent in using a hack saw. Since a hack saw is almost invariably available to a person who has occasion to cut shielded cable, the cost of a cable cutter must be low enough not to discourage its purchase as a supplement to the hack saw. Furthermore, a hack saw can be brought into action on a piece of shielded cable practically instantaneously. Therefore, the problem is not just to provide a cable cutter that is low in cost, functionally satisfactory and reasonably convenient in use, but to provide a cable cutter which offers the user so much speed and convenience that he has no incentive to use the hack saw instead.
In particular, the means for securing the cable in the cable cutter must be operable with extreme speed, ease and simplicity. In this respect, this securement means must adapt itself automatically to cable of any commercial diameter without the need for making any adjustment. Nevertheless, it must assuredly prevent the cable from being displaced either laterally or longitudinally by the action of the cutter upon it. With all of this, the device as a whole must be sturdy, reliable and inexpensive.
As is apparent from the foregoing discussion of the prior art, the requirement for automatically accommodating cable of different diameters has heretofore presented an especially troublesome problem. In most cases there has been no attempt to solve it, and no satisfactory solution to it has come forward. This problem is complicated by the substantial variation in outside radius from point to point along the length of every conventional shielded cable and by the need for so fixing the cable in the cable carrier that a diametrical plane through its shielding will substantially coincide with the plane of the saw blade that cuts it, irrespective of the nominal diameter of the cable.
Thus, the prior art demonstrates that each of the several problems involved in providing a satisfactory shielded cable cutter has received individual attention, but skill in the art has not heretofore been able to bring forth a satisfactory solution to all of those problems in their complex relationship to one another. This failure is not due to lack of incentive. The devices of some of the above discussed patents have achieved a degree of commercial success, from which it is evident that there is potentially a very profitable market for a shielded cable cutter that not only satisfies the obvious need for low cost, sturdiness and safety but also affords no less convenience and speed than a hack saw seems to offer.